Based on the $(2712.4 \pm 14.4)\times10^{6}$ $\psi(3686)$ events collected with the BESIII detector, we present a high-precision study of the $\pi^{+}\pi^{-}$ mass spectrum in $\psi(3686)\to\pi^{+}\pi^{-}J/\psi$ decays. A clear resonance-like structure is observed near the $\pi^{+}\pi^{-}$ mass threshold for the first time. A fit with a Breit--Wigner function yields a mass of $(285.6 \pm 2.5)\,\mathrm{MeV}/c^{2}$ and a width of $(16.3 \pm 0.9)\,\mathrm{MeV}$ with a statistical significance exceeding $10\sigma$. To interpret the data, we incorporate final-state interactions (FSI) within two theoretical frameworks: chiral perturbation theory (ChPT) and QCD multipole expansion (QCDME). ChPT describes the spectrum above $0.3\,\mathrm{GeV}/c^{2}$ but fails to reproduce the threshold enhancement. In contrast, the QCDME model---assuming the $\psi(3686)$ is an admixture of $S$- and $D$-wave charmonium---reproduces the data well. The pronounced dip near $0.3\,\mathrm{GeV}/c^{2}$ offers new insight into the interplay between chiral dynamics and low-energy QCD.
Observation of a Threshold Enhancement in the π + π − Spectrum in ψ(3686) → π + π − J/ψ Decays
Garzia, I.;Melendi, F. M.;
2026
Abstract
Based on the $(2712.4 \pm 14.4)\times10^{6}$ $\psi(3686)$ events collected with the BESIII detector, we present a high-precision study of the $\pi^{+}\pi^{-}$ mass spectrum in $\psi(3686)\to\pi^{+}\pi^{-}J/\psi$ decays. A clear resonance-like structure is observed near the $\pi^{+}\pi^{-}$ mass threshold for the first time. A fit with a Breit--Wigner function yields a mass of $(285.6 \pm 2.5)\,\mathrm{MeV}/c^{2}$ and a width of $(16.3 \pm 0.9)\,\mathrm{MeV}$ with a statistical significance exceeding $10\sigma$. To interpret the data, we incorporate final-state interactions (FSI) within two theoretical frameworks: chiral perturbation theory (ChPT) and QCD multipole expansion (QCDME). ChPT describes the spectrum above $0.3\,\mathrm{GeV}/c^{2}$ but fails to reproduce the threshold enhancement. In contrast, the QCDME model---assuming the $\psi(3686)$ is an admixture of $S$- and $D$-wave charmonium---reproduces the data well. The pronounced dip near $0.3\,\mathrm{GeV}/c^{2}$ offers new insight into the interplay between chiral dynamics and low-energy QCD.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
